1. Field of the Invention
The invention relates to an integrator-filter circuit for filtering a push-pull signal, comprising at least two integrator elements, each of said integrator elements having resistors arranged at its inputs, a subsequent current multiplier having two signal inputs and preceding a push-pull amplifier with an inverting input and a non-inverting input, the inverting output being fed back to the non-inverting input and the non-inverting output being fed back to the inverting input via respective capacitances, the current multiplier receiving, at two control inputs, a first and a second control current by means of which the integration time constant of the integrator element is adjustable and from which the second control current flows in substantially two halves through the signal inputs of the current multiplier.
2. Description of the Related Art
Such integrator-filter circuits are known from, for example, Philips-IC TDA 9715 in which they are used, for example, for chrominance subcarrier filtering in a video signal. Such filters are also known from "Integrated Continuous-Time Filter", Y. P. Tsinidis, J. O. Voorman, IEEE Press, 1992/93.
Such integrator-filter circuits are particularly typical in that their filter frequency is adjustable by means of the first and the second control current. To this end, a plurality of integrator elements are arranged appropriately within the integrator-filter circuit. Each integrator element incorporates a push-pull amplifier which is preceded by a current multiplier, which is also referred to as Gilbert multiplier. The outputs of the push-pull amplifier are fed back to its inputs via capacitances. The current multiplier is preceded by resistors. The control currents are applied to the current multiplier at control inputs. By variation of the control currents, the impedance at the input of the push-pull amplifier can be varied. Consequently the time constant or integration time constant is varied.
The drawback of such integrator-filter circuits, and the integrator elements therein particularly, consists in that the second control current also flows into the inputs of the current multiplier. The control current occurs in substantially two halves at the two inputs. Since this control current thus also flows through the resistors preceding the current multiplier, the control range which the signal to be filtering may have is influenced by the value of the control currents. Accordingly, the control range is reduced at a small control current.